Because you have to ask what benefit will it serve in exchange for the effort? In places werwere it really matter we already do, and conversions are pretty simple otherwise. Sometimes fractional units are just slightly easier for a specific task, and having used them our whole lives they are second nature.

To me asking why we don't have a single measuring standard is similiar to asking why we don't all agree on a single language. Sometimes it would be easier, sometimes it wouldn't, but in the end it doesn't matter all that much.